The Pittsburgh left is a colloquial term for the driving practice of the first left-turning vehicle taking precedence over vehicles going straight through an intersection, associated with the Pittsburgh, Pennsylvania area. The practice is also referred in various locales as a Boston left,[1]Massachusetts left, Rhode Island left, Jersey left (not to be confused with a jughandle), and New York left.[2] It is an illegal[3] and controversial[4] practice.

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The Pittsburgh left involves two cars facing one another waiting at a traffic light or other stop signal: one turning left and one going straight. (The signal in question has no dedicated left-turn advance green.) The left-turning car will anticipate the green light, although ideally not actually entering the intersection on the red.

As the light turns green, the left-turning car will rapidly proceed left ahead of the opposing traffic, frequently with the signaled assent of the opposing traffic. By accepting a modest delay in going straight, the opposing traffic has saved the left-turning traffic waiting an entire light cycle to turn left, as well as saved an equivalent amount of time for all the cars that otherwise would have been stuck behind the left-turning car. In situations where there is so much oncoming, straight traffic that a left turn would not be otherwise possible during any part of the light cycle, the Pittsburgh left can allow a line of left turning traffic to proceed incrementally.

As this modest delay is incurred by the opposing driver, with the hopes of similar treatment at some later date, it represents reciprocal altruism on the part of all participants.

Frequently, the on-coming cars will accelerate slowly enough to allow the turn to be completed without anyone slowing down or being delayed at all. This means that a "Pittsburgh left" can, in places other than Pittsburgh, be the equivalent of simply judging the oncoming traffic to be slow and distant enough to turn in front of it: something that in and of itself is not illegal.

Generally, a wave of the hand in the direction of the turn or a flashing of headlights by the driver going straight will indicate permission for the left-turning driver to execute the turn. Conversely, the driver navigating the turn may attempt to signal the other driver for permission to do so, with similar hand motions or headlight signals. Furthermore, simply a delay by the straight-bound car can often initiate the signal, and give the left turner time to go and make the turn, especially in large intersections.[citation needed]

Before the mid-1980s, Pittsburgh had a non-standard traffic light sequence. The green light would become simultaneous green/yellow, then just yellow, and finally red.[5] While some believe this sequence reduced the chances of cross-traffic running the red light immediately before the anticipated green, and thus encouraged early left turning, this practice is also observed in other regions with no history of simultaneous green and yellow. Some large intersections have four-way red for several seconds as a safety buffer, providing an opportunity for an illegal and unsafe left turn after cross-traffic has physically stopped but before the green light gives legal right of way.

Also, many of Pittsburgh's roads are narrow, one-lane streets, and if a car is attempting to turn left, no other cars behind it can proceed through the intersection. Allowing the left-turning car to proceed first through the intersection, allows all traffic to proceed normally with minimal disruption.